Ion channels are proteins which regulate the passage of ions across cell membranes (Hille, Ionic Channels of Excitable Membranes Sunderland, Mass., Sinauer Associates (1984)). As ions carry charge, ion channels are important mediators of fundamental cell electrical properties, including the cell resting potential. Ion channel activity is defined as the passage of ions through the channel. The activity of ion channels regulates dynamic electrical events such as depolarization and hyperpolarization. Such dynamic cellular electrical events are critical for a wide variety of physiological processes, including neuronal signaling, control of heart rate, muscle contraction and secretion. Thus, treatments which affect the activity of ion channels, in turn, regulate many physiological processes.
The importance of ion channels for normal physiological function is underscored by studies of mutations in genes which code for ion channels. Such mutations have been implicated in pathological states. In particular, mutations in ion channel genes expressed in the heart result in long QT syndrome, a potentially fatal cardiac arrhythmic disorder (Keating, M. T., et al., Curr. Opin. Gen. Dev., 6: 326-333 (1996); Deal, K. K., et al., Physiol. Revs., 76: 49-67 (1996); Keating, M. T., et al., Science, 272: 681-685 (1996)), and central nervous system ion channel mutations result in episodic ataxia (Adleman, J. P., et al., Neuron., 15: 1449-1454 (1995)). Mutations in skeletal muscle ion channels result in movement disorders. For example, mutation of the KCNA gene is associated with myokymia, a condition characterized by sudden loss of coordination and tremor of the head and limbs (Gutmann, L. et. al., Neurology, 47: 18-21 (1996)).
Pharmacological studies provide further evidence for the involvement of ion channels in pathological disorders. The anti-histamine drugs terfenadine and astemizole can induce long QT syndrome and cardiac arrhythmia (Woosley, R. L., Annu. Rev. Pharmacol. Toxicol., 36: 233-252 (1996)). The mechanism of toxicity of these drugs is due to cardiac potassium channel blockage, including potassium channels which are disrupted in the inherited cardiac arrhythmia disorder described above (Keating, M. T., et al., Science, 272: 681-685 (1996)). Myokymia can be induced by 4-aminopyridine, a potassium channel blocker (Gutmann, L. et. al., J. Neurology, 47: 18-21 (1996)). Although these studies provide insights towards understanding the mechanisms underlying these diseases, inherited and drug-induced pathologies (Keating, M. T., et al., Science, 272: 681-685 (1996)) account for a small minority of total cases. Cardiac arrhythmia is associated with most of the 250,000 sudden cardiac deaths in the United States each year. However, only a small of fraction of the affected individuals have inherited cardiac arrhythmia or suffer from drug-induced pathology. Thus, it is necessary to search for other causes of ion channel blockage which would contribute to such pathologies.
There is, therefore, a need for an improved understanding of mechanisms of potassium channel blockage. Also needed are methods of assessing the ability of a potential therapeutic agent to inhibit the blockage of a potassium channel.